US1830240A - Electric wave limiting device - Google Patents

Description

Nov. 3, 1931. E. PETERSON 1,830,240

' ELECTRIC WAVE LIMITING DEVICE Filed May 18, 1929 m4 5k 2 2 war? E6 8 jfi/a g 598 /9 g f :li' (23 :llll I2? aurPz/T Y MA/v 30 [I] 3 38 Q llvPl/T 5 33 1 33 39 II M534 wmv 701? E. PETERSON A r ram av Patented Nov. 3, 1931 PATENT OFFICE EUGENE PETERSON. OF NEW YORK, N. Y.,

ASSIGNOB TOIBELL TELEPHONE LABORA- TORIES, INCORPORATED, OF NEW YORK, N. Y., A' CORPORATION OI NEW YORK ELECTRIC WAVE LIMITING DEVICE Application filed May 18, 1929. Serial No. 364,226.

This invention relates to wave transmission systems and more particularly to means for limiting disturbances in amplifier circuits.

In order to obtain high efficiency and maximum power from high power amplifiers it is desirable to keep the input voltage as large as possible without overloading the tubes or producing excess distortion. Provision must be made, however, for transients or other unavoidable disturbances which when they occur might produce undesirable results.

An object of the invention is to limit the peak voltages impressed upon a vacuum tube amplifier which are in excess of normal signal voltages.

Another object of the invention is to produce a sharp or abrupt limitation of both positive and negative voltage peaks.

In accordance with the present invention two amplifier tubes are connected in cascade. Each tube has a resistance in the input circuit which prevents any increase in the plate current when the potential upon the grid of the tube exceeds a predetermined value. When a Wave is impressed upon the voltage limit ing device, it is faithfully re eated until a certain value is reached beyon which any increase in potential of either polarity is unaccompanied by further increase of output current. The two stages in cascade are connected so that limitation of one polarity takes place in the first stage while the other polarity is limited in the succeeding stage. The circuit thus permits both halves of the wave to have grid circuit limitations which results in much sharper cut-off than if plate circuit A better understanding of the invention may be had from the following description and the accompanying drawings, of which .Figs. 1 to 3 are difierent circuits embodying the invention and Figs. 4 to 6 are curves illustrating their operation.

Referring to Fig. l, a pair of vacuum tubes 1 and 2 having similar input and output circuits are seen connected in cascade through an interstage transformer 3. The input c1rcuit of the tube 1 may be traced from the grid 4 through resistance 5, the secondary Winding 6 ot' the input transformer 7, and

the source of grid potential 8 to the filament 9 of the tube. The output circuit of tube 1 may be traced from the late 10 through primary winding 11 of tie interstage transformer 3 and a source of plate potential 13 to the filament 9 of the tube. Similar circuits may be traced for the vacuum tube 2. Thus, the input circuit of the tube 2 may be traced from the grid 14 through the resistance 15, the secondary winding 16 of the transformer 3, and the source of grid potential 17 to the filament 18 of the tube. The output circuit may be traced from the plate 19 through the primary winding 20 of the output transformer 21, and the source of plate potential 22, back to the filament circuit 18. The filaments 9 and 18 are energized, respectively, by the sources of energy 23 and 24.

The operation of the circuit may be readily understood by reference to Fig. 4. The curve a is the grid-potential plate-current characteristic of one stage of the amplifier. When a normal voltage wave, such as that repre; sented by b is applied to the grid of tube 1, it is amplified and appears as a current wave 0 in the plate circuit of the tube. When a voltage wave, represented by d, exceeds a certain value determined by the biasing potential on the grid, the latter element becomes positive and grid current flows. However, when the grid is positive, the internal resistance between filament and grid is low, and resistance 5 is so large compared with it that practically all of the potential drop occurs across this resistance, the grid potential going only slightly above its zero value. For th.s reason the plate current does not follow the variations of the input wave when the positive cycle of the latter exceeds the negative grid potential 8 but is chopped off as shown by curve e. The same phenomenonbccurs in the input circuit of. the tube 2, but the windings of the interstage transformer 3 are poled so that in this circuit the other half cycle of the wave is limited.

Fig. 5 illustrates the phenomenawhich take place in different parts of the circuit when the input wave is in excess of the normal value. Thus G represents the input voltage as a sine wave havin peak values greatl in excess of the grid biasing voltage, whic is represented by E This wave is cut off in the manner heretofore described so that the voltage impressed on the grid of tube 1 is shown by H. 'The voltage appearing at the high side of the transformer 3 has the DE component removed and is shown by I. This wave has the same shape as that shown by II, but is moved up so that the area of the positive half cycle of the wave is .e ual to the area of the negative half cycle. 11 account of the amplification occurring in tube 1, this wave, I, would have a greater amplitude than the grid wave, H, but for'simplicity in explaining the action, the effect of the amplification has been neglected in these curves. The voltage Wave impressed upon the grid of the tube 2 with the negative portion of the wave limited is shown b J. The portion of the wave that is limite in this stage is the negative portion of the original input wave although as regards the tube 2 the part of the wave limited in its grid circuit is positive on account of the shift in phase occasioned by the transformer 3. The final output wave is represented by K and has the DC component introduced in the plate circuit of the tube 2 removed.

In order to repeat waves of the shapes indicated in Fig. 5, the interstage and output transformers of Fig. 1 should have good transmission characteristics over the frequency band including all of the important components represented by the fiat topped waves, and should also have a zero or a small phase shift over this range.

In the case of a radio transmitter, the power limiter of the invention would-be used to prevent damage to the high power transmitting tubes by preventing excessive voltages from reachin them through the input circuit. The circuit ofFigs. 1, 2 or 3 would be placed in the input circuit leading to the high power tubes. In this case the antenna would preferably have a relatively-narrow band frequency characteristic, incapable of transmitting the wide range of components which the transformers of Fig. 1 (or t e Figs. 2 and 3 circuits) are designed to transmit, and this will reduce to a harmless amount the distortion arising from the flattening of the peaks of the waves as shown in Figs. 5 and 6. v

Fig. 2 shows another embodiment of the invention in which the two tube stages are resistance coupled, thereby permitting both halves of the waves to be limited without removing the DC components. This circuit permits both halves of the waves to be cut off in exactly the same manner. In Fig. 2 the input of the tube 30 is the same as the input circuit of tube 1 of Fig. 1, while the output circuit of tube 31,is similar to the output circuit of tube 2. The output circuit of tube 30 may be traced from the plate 32 through the resistance 33, the source of plate potential 34- to the filament 35. The input circuit'of tube 31 may be traced from the grid 36, through the resistance 37, source of grid potential 38, the resistance 33, and source of potential 34 to the filament 39.

In the operation of this circuit the potential which appears in the output of the tube 30 is opposite in phase to the voltage impressed upon its grid. Thevoltage appearing across resistance 33, containing a DO component from the plate circuit of tube 30, is limited in the input circuit of the tube 31. By means of this circuit both halves of the wave may be limited by precisely the same amount thus giving a somewhat better result than that obtained by the. circuit of Fig. l.

The phenomena which take place in different parts of this circuit are illustrated by Fig. 6. For instance, L represents the input voltage as a sine wave. After limitation occurs in the input circuit of the tube 30 in the manner heretofore described, the voltage impressed upon its grid is shown by M. The voltage in the output circuit of this tube due to the voltage upon its grid is shown by Nand is exactly the same shape although opposite in phase to M. When limitation is effected in the grid circuit of tube 31 the resultant voltage is shown by O and since the DC component from theplate circuit of the tube 30 waspresent at the time that limitation was effected both half cycles of the resultant wave are equal and symmetrical. The wave appearing in the output of this circuit is shown by P. In the case just described the resistance 33 of Fig. 2 may be apportioned so that the input levels of tubes 30 and 31 are equal.

Fig. 3 shows another type of voltage limiter also employing resistance coupling. In this circuit an attenuating circuit 50 is connected between the tube stages, in order that the AC voltage applied to the grid of the second tube may exactly equal that applied to the first tube. This permits the limitation of both halves of the'wa-ve to be effected at zero gain, so that both halves of the waves are tubes 53 and 55 is effected in the same way as in the circuits heretofore described. The curves of Fig. 6 are equally applicable to the said limiting operation of the circuit of Fig. 3 and of the voltages impressed upon the input circuit' of said devices from exceeding a predetermined value.

2. A voltage limiting device com rising space discharge devices in a plura ity of stages, a resistance in the input circuit of each of said devices for limiting only the positive voltage peaks impressed upon the input circuits thereof to a predetermined value, and. means for associating said tubes with each other'in such a manner that both peaks of the voltage wave impressed upon device are subjected to the limiting action.

. 3. An electric wave limiting device comprisin grid control space discharge devices m a p urality of stages, a resistance in the input circuit of each of said devices for limiting only the positive wave peaks impressed upon said input *circuits and means for connecting said space discharge devices in such a way that the wave is impressed upon the grids of said devices in phase opposition.

' 4. An electric wave limiting device com-" prising grid control space discharge devices in a p urality of stages, a source of bia sin gotential in the input circuit of each of sai evices, means comprising a resistance for preventing the grids of said tubes from becoming positive when the wave applied to the input 0 said limiting device exceeds a redetermined value, and interstage coup ing means. for said space discharge devices for permitting that portion of a wave to be limited in one stage which was not limited in a preceding stage.

5. In a voltage limiting device. a pair of vacuum tubes connected in tandem relation, means in the input circuit of one of said tubes for dproviding a negative .potential upon the gri with respect to the cathode thereof, means for impressing voltage waves upon said input circuit, means therein for. reventing said grid from becoming positive with respect to said cathode, correspondin "means in the input circuit of the other of sai tubes, and means for impressin the volta wave as limited by said first tu upon t e in at circuit of said second tube in oppositepo arity to said tube.

ing a value sufiicient to the voltage 6. In a voltage limiting device, a pair of vacuum tubes connected in tandem relation, means in the input circuit of one of said tubes for providing a negative potential upon the grid with respect to the cathode thereof, means for impressing a voltage wave upon said input circuit, a resistance therein havrovide a negative grid bias when current ows in said input circuit equivalent to the voltage by which the positive voltage wave exceeds the absolute value of said first mentioned grid bias, and corresponding means in the input circuit of the other of said tubes, and means for impressing the voltage wave as limited by said first tube upon the input circuit of said second tube in opposite phase relation to said 7. In a communication channel, a eak voltage limiting device comprising a t reeelectrode vacuum tube having in the input circuit thereof a resistance man times as great as the input impedance 0 said tube at zero grid bias and a negative grid biasing voltage approximately equal to the maximum voltage of the signals normally transmitted, and a second voltage limiting device in tandem therewith, poled to limit eaks not subject to the limiting action 0 said first device.

8. In a' signal channel, a voltage limiting circuit comprising a three-electrode vacuum tube having an input circuit com rising a resistance large in comparison wit the input impedance of said tube when grid current starts to flow, and a grid biasing ,volta proportioned to the maximum voltage to carried normally by said channel to limit the positive peaks transmitted to substantially said maximum normal voltage, and a second voltage limiting circuit in tandem therewith poled to limitthe peaks not subject to the limiting action of said first circuit.

- first tube.

In witness whereof, I hereunto subscribe

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